Typically, coated glass articles are produced by continuously coating a glass substrate while it is being manufactured in a process known in the art as the “Float Glass Process.” This process involves casting glass onto a molten tin bath which is suitably enclosed, then transferring the glass, after it has sufficiently cooled, to take-away rolls which are aligned with the bath, and finally cooling the glass as it advances across the rolls, initially through a lehr and thereafter while exposed to the ambient atmosphere. A non-oxidizing atmosphere is maintained in the float portion of the process, while the glass is in contact with the molten tin bath, to prevent oxidation. An air atmosphere is maintained in the lehr. The chemical vapor deposition (CVD) of various coatings may be conveniently performed in the bath or the lehr, or even in the transition zone therebetween, by contacting the surface of the hot glass with chemical vapor-containing reactants which pyrolytically decompose to form the metal oxide coating. This, of course, requires that the chemical reactants have vaporization temperatures below their thermal decomposition temperatures. A number of metal-containing compounds exist which may be vaporized to prepare a metal oxide coating on glass by CVD technology. Of particular interest are titanium compounds which are vaporized to form titanium oxide coatings.
The desirable performance characteristics of certain metal oxide coatings, such as titanium oxide coatings, on glass, for example, photocatalytic activity, hydrophilicity, high light transmittance, high infrared reflectivity, etc., can be improved by the incorporation of a dopant into the metal oxide coating. Among the materials used in the prior art as dopants for titanium oxide is nitrogen. Typically, nitrogen doped titanium oxide (TiO2:N) coatings and films have been prepared by physical vapor deposition (PVD) such as direct current magnetron sputtering etc. To be compatible with the float glass process, the coatings must be formed by chemical deposition methods, such as chemical vapor deposition (CVD), preferably atmospheric pressure chemical vapor deposition (APCVD).
The physical form of the reactants employed in glass coating processes is generally a liquid, solid, vaporized liquid or solid, liquid or solid dispersed in a carrier gas mixture, or vaporized liquid or solid dispersed in a carrier gas mixture. The chemical vapor deposition process generally employs a vaporized liquid or solid, which is typically dispersed in a carrier gas mixture.
It is further known to deposit thin coatings having one or more layers with a variety of properties onto glass substrates. One property of interest is photocatalytic activity which arises by photogeneration, in a semi-conductor, of a hole-electron pair when the semi-conductor is illuminated by light of a particular frequency. The hole-electron pair can be generated in sunlight and can react in humid air to form hydroxy and peroxy radicals on the surface of the semi-conductor. The radicals oxidize organic grime on the surface which both cleans the surface and increases the hydrophilic properties (i.e. wettability) of the surface. A hydrophilic surface is beneficial because water will wet the surface better, making the surface easier to clean with water containing little or no detergent. In addition, water droplets will spread over the surface reducing the distracting visual effects of rain or spray. Thus, photocatalytically active coated glass has a use in self-cleaning glass windows.
Commonly, TiO2 only becomes photocatalytic and hydrophilic upon irradiation with UV light (λ<400 nm). It is known that doping TiO2 with certain metals, and more recently, with anions as described in the publication “Visible Light Photocatalysis in Nitrogen-Doped Titanium Oxides,” Asahi, R. et al., Science, Vol 293, p. 269–271 (2001), can introduce a visible absorption band into the TiO2 which may lead to visible light activated photocatalysis. The thin films described in the Asahi, et al. article are formed by physical vapor deposition processes.
Titanium dioxide may be deposited onto glass to form a transparent coating with photocatalytic properties. In WO 98/06675, a chemical vapor deposition process is described for depositing titanium oxide coatings on hot flat glass at high deposition rate. In EP 901 991 A2, a photocatalytic active titanium oxide coating deposited by CVD is disclosed.
U.S. Pat. No. 6,306,343 describes a method of decomposing material by contacting the subject material with a photocatalyst exposed to light including visible light. The photocatalyst is said to be formed by subjecting TiO2 to plasma CVD treatment, where the plasma contains a mixed gas of a hydrocarbon and a reducing agent.
Published U.S. patent application Ser. No. 20020012779 A1 describes a multi-layer film of the form substrate/TiO2/TiO2—SiO2. This film stack is said to exhibit photocatalytic properties upon exposure to visible light.
Published U.S. patent application Ser. No. 20020151434 A1 describes a photo-catalyst comprising an oxynitride containing at least one transition metal wherein the transition metal is one chosen from the group, La, Ta, Nb, Ti and Zr, or Ca, Sr, Ba, K, and Rb. The oxynitride thus formed is said to be responsive to visible light.